A motor-vehicle drive is usually provided with a cooling fan which forces a current of air either directly over the engine or through a radiator forming part of the engine-cooling system. Since motor-vehicle engines, usually of the internal-combustion type, are of limited efficiency and produce enormous quantities of heat which must be dissipated, it is necessary that this cooling system have a relatively large capacity. Thus it is not at all uncommon in such engines for the cooling system itself to use between 10 and 20% of the power output of the motor.
It is known to reduce the energy consumption of the cooling system by providing a transmission between the blower and the engine. This transmission is controlled according to the temperature of the motor so that the quantity of air displaced is dependent on the engine-cooling requirements. The arrangement is traditionally connected so that, as the engine temperature increases, the fan speed of the cooling system increases, and therefore the capacity of the cooling system to dissipate this increased heat is augmented.
Such an arrangement limits the loading to which the engine can be subjected. This is particularly true when the engine has been operating for a relatively long period of time and is already quite hot as, in this circumstance, the fan will be driven at full speed practically all the time and will rob the engine of a predetermined proportion of available power whether or not this power could better be used otherwise.